The invention described herein was made in the performance of official duties by an employee of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon.
1. Field of the Invention
The invention relates generally to use of the Global Positioning System (GPS) to generate local position data, and more particularly to a self-contained, self-surveying differential GPS base station and method of operating same that automatically surveys the base station""s local position without any operator intervention.
2. Background of the Invention
Satellite-based global positioning systems such as the U.S.-based Global Positioning System (GPS) and the former Soviet Union""s GLONASS system are well known in the art. While these systems were initially developed for military applications, their use in a variety of civilian applications has steadily increased. For years, the U.S.-based GPS provided a higher degree of positional accuracy for military applications than for civilian applications owing to a deliberately-induced error factor known as xe2x80x9cselective availabilityxe2x80x9d.
Since the deactivation of xe2x80x9cselective availabilityxe2x80x9d on May 2, 2000, a standard off-the-shelf GPS receiver currently provides position information with an error of approximately 10 meters. A significant improvement in accuracy can be obtained through the use of differential corrections generated by a differential GPS (or DGPS) base station. Depending on the hardware, errors of less than 1 foot can be achieved. However, these highly accurate results can only be achieved if the base station is in close proximity to the operating area of rover DGPS receivers. Typically, the use of a publicly-available base station (e.g., a U.S. Coast Guard base station) does not produce highly accurate results because there is usually a large distance between the rover DGPS receivers and a U.S. Coast Guard base station. Accordingly, it is extremely beneficial to have a xe2x80x9clocalxe2x80x9d differential base station that is closer to rover receivers than one of the fixed-position, publicly-available base stations.
A differential base station operates by taking the known position of its GPS antenna and, in essence, reverse calculates what it should expect to see from the satellites. As the base station receives the satellite data, it compares the expected and the actual data received to compute corrections for each satellite in view. The computed corrections are then sent to rover DGPS units, which apply the corrections to their own position calculations to remove errors resulting from atmospheric conditions and timing errors with the satellites.
The setting up of a base station requires that the position of its GPS antenna be surveyed in order to enable the above described correction calculations. The base station""s GPS antenna must be surveyed with a high degree of accuracy to enable rover DGPS receivers to provide highly accurate absolute position information. Typically, the setting up of a base station consists of having a survey crew determine the latitude, longitude, and altitude of the station""s GPS antenna. Such manual surveying techniques require time, money and additional equipment. An alternative approach is to attach a PC to the base station and have it average the position information being returned by the station. The user must then take the calculated position from the PC and update the base station""s setup parameters. This method requires that a man be present on site for some period of time. Thus, neither of these methods can be done in stride or in a xe2x80x9cdrop-and-goxe2x80x9d fashion.
Accordingly, it is an object of the present invention to provide a self-contained, self-surveying differential GPS base station and method of operating same that will produce accurate position information for rover DGPS receivers simply by turning on the base station.
Another object of the present invention to provide a self-contained, self-surveying differential GPS base station for use in areas that cannot be surveyed by conventional means.
Still another object of the present invention to provide a method of operating a differential GPS base station.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a self-contained, self-surveying differential GPS base station system and method of operating same are provided. The system and method use a differential GPS base station capable of generating National Marine Electronics Association (NMEA) data sentences and capable of generating Radio Technical Commission for Maritime Services (RTCM) differential corrections for transmission over the air waves. Each NMEA data sentence identifies a determined position of the base station and fix quality information associated with the determined position. The system and method automatically identify the determined position as an acceptable position when the fix quality information associated with the determined position satisfies a selected criteria which is stored on-site. The system and method then automatically average together each acceptable position to generate an updated position. The updated position is automatically supplied to the base station at a prescribed time for use by the base station in generating the RTCM differential corrections.